1. Field of Invention
The present invention relates to semiconductor lasers and, more particularly, to wavelength beam combining applied to arrays of quantum cascade lasers.
2. Discussion of Related Art
Quantum cascade lasers (QCLs) are electrically driven semiconductor lasers. They are compact and can be efficient and reliable. QCLs emit or lase at room temperature in the infrared region at wavelengths spanning about 3 micrometers (μm) to 24 μm. This wavelength range overlaps many molecular absorption lines of interest. Individual QCLs can achieve watt-level output power in continuous-wave operation at room temperature and can be designed to have broadband gain such that the lasing wavelength can be tuned over a broad spectral range of approximately 300 cm−1.
Conventional diode lasers generate photons through an electronic transition across the bandgap of a semiconductor in which the diode laser is fabricated, as shown in FIG. 1. Accordingly, the emission wavelength is determined by the bandgap of the semiconductor material. In contrast, QCLs operate based on resonant tunneling and optical transitions between electronic levels within the conduction band of the semiconductor. The energy levels, and hence the lasing wavelength, can be controlled by modulating the electronic potential applied to the semiconductor. FIG. 2 illustrates an energy-band diagram of a generic QCL under bias. The applied electric field drives the electrons from the right toward the left, as indicated by arrows 210. The gain region includes a series of cascades 220 formed by alternating layers of two different semiconductor materials, each cascade including an injector region 230 and an active region 240. Electrons transit an injector region 230 and are injected into the upper lasing level within the active region 240. The electron undergoes a radiative transition to the lower lasing level, emitting a photon, and is then scattered into the ground state where it is coupled into the next injector region 230. Ideally, an electron can generate as many photons as there are cascades 220 in the gain region of the QCL.
Continuously-tunable, single-mode emission is required for many spectroscopic applications. To achieve single-mode emission, QCL material is generally processed into distributed feedback (DFB) lasers or integrated with a frequency-selective external cavity. External cavity QCLs are widely tunable (e.g., 10-20% of the laser emission frequency), but are cumbersome and complex to build as they require high quality anti-reflection coatings and well-aligned external optical components including a grating for tuning. Furthermore, wavelength tuning is relatively slow because it depends upon mechanical movement of the optical components. DFB QCLs include a grating defined in the waveguide for wavelength control. An example of a distributed feedback quantum cascade laser element is illustrated in FIG. 3. The width of the emitting aperture of the DFB QCL element is defined by etching into the semiconductor material. Current is generated through the active regions of the DFB QCL via a metal contact 310 on top of the ridge 320 and a substrate contact (not shown), which in turn generates optical gain in the DFB QCL. The optical power output 330 is emitted normal to the cleaved facet. In the illustrated example, the grating 340 is etched into the top layer of the semiconductor. The lasing wavelength is determined by the period of the grating, and wavelength tuning is achieved by changing the temperature of the laser. DFB QCLs are compact; however, a single DFB QCL has limited tunability, typically only about 5-10 cm−1, which is much smaller than the available gain bandwidth which is generally greater than 100 cm−1.
Examples of a broadly tunable single-mode laser source based on an array of DFB QCLs driven by a microelectronic controller are described in U.S. patent application Ser. No. 11/611,819 entitled “CONTINUOUSLY TUNABLE SINGLE-MODE QUANTUM CASCADE LASER SOURCES AND SENSORS” patented on Nov. 2, 2010 under U.S. Pat. No. 7,826,509, and in “Widely tunable single-mode quantum cascade laser source for mid-infrared spectroscopy,” Lee et. al, Applied Physics Letters 91, 231101 (2007), both of which are incorporated herein by reference in their entireties. The laser source includes an array of DFB QCLs with closely spaced emission wavelengths spanning the gain bandwidth of the QCL material, fabricated monolithically on the same chip and driven individually by a microelectronic controller. Each DFB QCL in the array has a DFB grating designed to target a slightly different emission wavelength, and the difference in the target emission wavelengths between adjacent (in frequency space) DFB QCLs in the array is smaller than the tuning range available with temperature tuning of the DFB QCLs. Therefore, by switching between the DFB lasers in the array and using temperature tuning of the individual lasers in the array, continuous spectral coverage can be obtained within the gain bandwidth of the QCL material.